multiple computers sharing an index
Probably what's happening is: because older versions of Lucene store
the lock file in the /tmp directory by default, multiple computers
sharing an index will be able to open multiple writers because they
have their own /tmp directories. They don't see each other's lock.
Newer versions of Lucene store the write lock in the directory
itself, which resolves this. You can also change the lock directory
to a shared mount point.
Multiple indexers on same storage location in Lucene
If you suspect that this or any other LockFactory is not working properly in your environment, you can easily test it by using VerifyingLockFactory, LockVerifyServer and LockStressTest.